The Hog Louse 659 



around, passing into the second pair of legs. The fifth branch leaves the 

 plexus almost at the same point as the preceding, turns back, and enters 

 the main trunk centrad of the point of issue of the tracheae of the third 

 pair of legs, thus forming a loop, which may correspond to the thoracic 

 tracheal triangle described by Harrison (1916 a: 105) in some Mallophaga. 

 There, however, the thoracic stigma forms the apex of the triangle, while 

 this loop lies behind the stigma. Harrison suggests that the inner side 

 of the triangle may be the only survival of wing tracheae. The sixth 

 branch comes from the branch to the thoracic spiracle just dorsad of its 

 entrance into the main trunk, and passes into the second pair of legs. 

 As has been shown, two tracheae pass into each leg, one of which lies 

 ventral and the other dorsal. In the coxae, branches are given off which 

 break up into many fine tracheoles; in the femur a large branch is given 

 off from each trachea, and one of these branches passes along with the 

 main branches into the tibia, where the latter subdivide many times, 

 passing into the spur, the pad, the tarsus, and the claw. 



The main trunks, on leaving the tracheal plexus, bend centrad and 

 dorsad, passing into the head on either side of the esophagus and the aorta 

 directly under the occipital apoderae. Just behind the sub-esophageal 

 ganglion they diverge, and shortly give off a lateral branch to the neigh- 

 boring muscles. Behind the brain a branch is given off centrad, and from 

 its root the commissure of the sub-esophageal ganglion issues, while it 

 passes forward close to the lateral borders of the brain. The main trunks 

 continue forward alongside the antennal nerves, give off a branch to each 

 antenna, and break up into numerous branches among the glands, the 

 fat cells, and the sensory cells of the anterior region of the head. 



The external surface of the stigma resembles a cart wheel with an open 

 hollow axis, and sections show the vestibule between the stigma and its 

 trachea to be filled with hair-like, chitinous structures radiating from its 

 inner surface to a thin wall surrounding a slender central canal (Plate LIX, 

 3). These spoke-like projections doubtless prevent the entrance of foreign 

 particles along with the air. A similar structure has been described by 

 Miiller (1915:30) in the clothes louse. Between the vestibule and the 

 trachea is inserted the closing apparatus, concerning the mechanism of 

 which there is still some uncertainty. Kranchcr (1881:522-533) briefly 

 described the structure in Hucmatopinus suis. His figure shows the 

 nature of the vestibule, the closing lever, and one intrinsic muscle between 



